A liquid crystal display device is a display device utilizing electro-optic changes of liquid crystal. Attention has been drawn to its characteristic such as small lightweight of the device or small consumption of electric power, and it has been developed significantly as a display device for various kinds of displays. Among these, a field effect type liquid crystal display device of twisted nematic type (TN type) is a typical example, wherein nematic liquid crystal having positive dielectric anisotropy is used, liquid crystal molecules are aligned in parallel to a substrate at the interface of each of a pair of electrode substrates facing each other, and the substrates are arranged so that the alignment directions of liquid crystal molecules are perpendicular to each other.
With regard to such a liquid crystal display device of TN type, it is important to align liquid crystal molecules so that the major axis direction of liquid crystal molecules is uniformly parallel to the substrate surface, and further liquid crystal molecules have a certain tilt angle to the substrate.
As typical methods to align liquid crystal molecules, two methods have been known. One method is to form an inorganic film on a substrate by vapor-depositing an inorganic substance such as silicon dioxide obliquely to the substrate, and to align liquid crystal molecule in the direction of vapor deposition. By this method, a stable alignment having a certain tilt angle can be obtained, but it is not industrially effective. The other method is to form an organic coating film on a substrate surface, to rub the surface with a cloth such as cotton, nylon or polyester in a predetermined direction, and to align liquid crystal molecules in the rubbing direction. By this method, stable alignment can be obtained relatively easily, and thus this method is mainly employed industrially. As the organic film, a polyvinyl alcohol, a polyoxyethylene, a polyamide or a polyimide may, for example, be mentioned. However, in view of chemical stability and thermal stability, a polyimide is most commonly used. A typical example of the polyimide used for a liquid crystal alignment film is disclosed in JP-A-61-47932.
On the other hand, with regard to a ferroelectric liquid crystal display device or an antiferroelectric liquid crystal display device, wherein ferroelectric liquid crystal or antiferroelectric liquid crystal having chiral smectic phase is used, and liquid crystal molecules are aligned at the interface of each of a pair of electrode substrates facing each other so that the layer direction is oriented in one direction to the substrate, it has been known to have more excellent high speed response and high angle of visibility as compared with a nematic liquid crystal display device, due to direct interaction between the field and a spontaneous polarization of liquid crystal molecules. Further, with regard to a surface-stabilized ferroelectric liquid crystal device or a surface-stabilized antiferroelectric liquid crystal device wherein the cell gap is made thinner than a helical pitch of ferroelectric liquid crystal or antiferroelectric liquid crystal, it has been known that a display device having bi-stability or tri-stability which can not be attained in a nematic liquid crystal display device and being highly precise even in a simple matrix electrode structure, can be prepared.
It is particularly important to align liquid crystal molecules uniformly in a predetermined direction in the ferroelectric liquid crystal device or the antiferroelectric liquid crystal device. As a typical method to align liquid crystal in such a manner, like in the case of the nematic liquid crystal display device, a method to form an inorganic film on a substrate by vapor-depositing an inorganic substance such as silicon dioxide obliquely, or a method to form an inductive coating film on a substrate surface and to rub the surface with a cloth in a predetermined direction, has been known. As the organic film to be rubbed, a polyimide is usually used, in view of chemical stability and thermal stability, like in the case of the nematic liquid crystal display device. However, with the conventional polyimide, the alignment of ferroelectric liquid crystal or antiferroelectric liquid crystal was not necessarily satisfactory. Further, even if an excellent alignment could be obtained, there were problems in the synthesis of the polyimide, such being problematic for the practical use.
With regard to the field effect type liquid crystal display device of TN type wherein nematic liquid crystal having positive dielectric anisotropy is used, liquid crystal molecules are aligned in parallel to a substrate at the interface of each of a pair of electrode substrates facing each other, and the substrates are arranged so that the alignment directions of liquid crystal molecules are perpendicular to each other, it is important to align liquid crystal molecules on the substrate surface, so that the major axis direction of liquid crystal molecules is uniformly parallel, and further, liquid crystal molecules have a certain tilt angle to the substrate. Particularly, in recent years, it is required to constantly obtain a low tilt angle of at most 2.degree., in order to improve the contrast of the field effect type liquid crystal display device of TN type.
However, with regard to a liquid crystal alignment film comprising a conventional polyimide, even if a low tilt angle was obtained after injection of liquid crystal, such a tilt angle sometimes changed when it was heated at a temperature higher than the isotropic temperature of liquid crystal (hereinafter referred to as isotropic treatment). Further, there was a problem that the tilt angle was likely to decrease and alignment of liquid crystal molecules was in disorder by isotropic treatment. Such a problem must be solved in order to obtain more uniform liquid crystal display having higher contrast for liquid crystal display devices, and a polyimide alignment film which gives a low tilt angle of at most 2.degree. constantly against heat treatment, has been desired.
On the other hand, with regard to a ferroelectric liquid crystal device or an antiferroelectric liquid crystal device, it is very important to obtain an uniform initial alignment of liquid crystal, and it has been known that the alignment condition significantly affects performance of a liquid crystal device. However, it is difficult to control the alignment condition of ferroelectric liquid crystal or antiferroelectric liquid crystal uniformly. Defects in alignment such as zigzag defects are usually observed on the rubbed polyimide film, and they cause a significant decrease in performance of the liquid crystal device such as a decrease in contrast, such being problematic. Further, as an alignment film having ferroelectric liquid crystal aligned well, an aliphatic diamine having an alkylene group in the main chain can be used, as disclosed in JP-A-8-248424. However, such an aliphatic diamine is poor in reactivity for polymerization, and there were problems in respect to obtaining a predetermined polyimide varnish.
It is an object of the present invention to provide an alignment treating agent for a liquid crystal cell using nematic liquid crystal, wherein a liquid crystal alignment film having a low tilt angle being stable against heat treatment, and an uniform liquid crystal display with high contrast can be obtained; and an alignment treating agent for a liquid crystal cell using ferroelectric liquid crystal or antiferroelectric liquid crystal, which controls the alignment condition of liquid crystal uniformly.